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        </a></span></div></div> <div data-v-0b87bb44><main class="page"><!----> <div class="page-title" style="display:none;"><h1>ArrayList底层实现和原理</h1> <hr> <div data-v-7b2e794a><i class="iconfont reco-account" data-v-7b2e794a><span data-v-7b2e794a>flybird</span></i> <i class="iconfont reco-date" data-v-7b2e794a><span data-v-7b2e794a>2017-06-13 20:53:26</span></i> <i class="iconfont reco-eye" data-v-7b2e794a><span id="/views/java/ArrayList.html" data-flag-title="Your Article Title" class="leancloud-visitors" data-v-7b2e794a><a class="leancloud-visitors-count" style="font-size:.9rem;font-weight:normal;color:#999;"></a></span></i> <i class="iconfont reco-tag tags" data-v-7b2e794a><span class="tag-item" data-v-7b2e794a>
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<span class="token comment">/**
 * Created by admin on 2017/6/10.
 */</span>
<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">MyArrayList</span> <span class="token punctuation">{</span>

    <span class="token comment">//ArrayList有一个专门用来装元素的容器，为了保证集合什么类型的数据都可存储，所以定义的Object[]</span>
    <span class="token keyword">public</span> <span class="token class-name">Object</span><span class="token punctuation">[</span><span class="token punctuation">]</span> data<span class="token punctuation">;</span>
    <span class="token comment">//ArrayList里还有一个属性，用来记录集合元素的个数</span>
    <span class="token keyword">public</span> <span class="token keyword">int</span> size<span class="token punctuation">;</span>

    <span class="token comment">/**
     * 有参构造方法
     *
     * @param x 指定数组大小
     */</span>
    <span class="token keyword">public</span> <span class="token class-name">MyArrayList</span><span class="token punctuation">(</span><span class="token keyword">int</span> x<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>x <span class="token operator">&gt;</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            data <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">[</span>x<span class="token punctuation">]</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
            <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;参数异常&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>

    <span class="token comment">/**
     * 如果指定数组大小，默认10个
     */</span>
    <span class="token keyword">public</span> <span class="token class-name">MyArrayList</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">this</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token comment">/**
     * 得到集合的大小  如：int x = list.size();
     *
     * @return
     */</span>
    <span class="token keyword">public</span> <span class="token keyword">int</span> <span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> size<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token comment">/**
     * 往集合中添加元素 如：list.add(Object obj);
     *
     * @param obj
     */</span>
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">add</span><span class="token punctuation">(</span><span class="token class-name">Object</span> obj<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token comment">//当我们往集合中添加元素的时候，obj最终都会添加进Object[]数组中去</span>
        <span class="token comment">//所以每次添加数据的时候都需要判断Object[],即data数组有没有填满</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>data<span class="token punctuation">.</span>length <span class="token operator">==</span> size<span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token comment">//如果填满了，那么需要扩容:</span>
            <span class="token comment">// jdk1.7之前：size*3/2+1</span>
            <span class="token comment">// jdk1.7及之后：size+(size&gt;&gt;1)</span>
            <span class="token class-name">Object</span><span class="token punctuation">[</span><span class="token punctuation">]</span> temp <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">[</span>size <span class="token operator">+</span> <span class="token punctuation">(</span>size <span class="token operator">&gt;&gt;</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
            <span class="token comment">//扩容后将老数组复制到新数组里</span>
            <span class="token class-name">System</span><span class="token punctuation">.</span><span class="token function">arraycopy</span><span class="token punctuation">(</span>data<span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> temp<span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> size<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token comment">//改变引用指向 gc回收老数组对象</span>
            data <span class="token operator">=</span> temp<span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
        
        data<span class="token punctuation">[</span>size<span class="token punctuation">]</span> <span class="token operator">=</span> obj<span class="token punctuation">;</span>
        size<span class="token operator">++</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token comment">/**
     * 按指定下标删除集合中的元素
     *
     * @param index
     */</span>
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">remove</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token comment">//System.arraycopy(要被复制的老数组，从下标index开始复制，要复制到的新数组，从新数组的下标index插入,从老数组下标开始要被复制的个数);</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span><span class="token function">arraycopy</span><span class="token punctuation">(</span>data<span class="token punctuation">,</span> index <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">,</span> data<span class="token punctuation">,</span> index<span class="token punctuation">,</span> size <span class="token operator">-</span> <span class="token punctuation">(</span>index <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        size<span class="token operator">--</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token comment">/**
     * 指定元素删除集合中的元素
     *
     * @param obj
     */</span>
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">remove</span><span class="token punctuation">(</span><span class="token class-name">Object</span> obj<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token comment">//每当指定元素删除的时候，底层会拿着obj和每个元素做equals比较</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> x <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> x <span class="token operator">&lt;</span> size<span class="token punctuation">;</span> x<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>obj<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>data<span class="token punctuation">[</span>x<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                <span class="token function">remove</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//按下标删除元素</span>
                <span class="token keyword">break</span><span class="token punctuation">;</span><span class="token comment">//一个remove方法只能删除一个对象</span>
            <span class="token punctuation">}</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>

    <span class="token comment">/**
     * 根据指定下标获得元素
     *
     * @param x
     * @return
     */</span>
    <span class="token keyword">public</span> <span class="token class-name">Object</span> <span class="token function">get</span><span class="token punctuation">(</span><span class="token keyword">int</span> x<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> x <span class="token operator">&gt;=</span> <span class="token number">0</span> <span class="token operator">&amp;&amp;</span> x <span class="token operator">&lt;</span> size <span class="token operator">?</span> data<span class="token punctuation">[</span>x<span class="token punctuation">]</span> <span class="token operator">:</span> <span class="token string">&quot;参数越界异常&quot;</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

<span class="token punctuation">}</span>

<span class="token keyword">class</span> <span class="token class-name">TestMyArrayList</span> <span class="token punctuation">{</span>
    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">MyArrayList</span> myArrayList <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">MyArrayList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        myArrayList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token number">234</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        myArrayList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">&quot;john&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        myArrayList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">&quot;demon&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        myArrayList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">&quot;alex&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>myArrayList<span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>myArrayList<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        myArrayList<span class="token punctuation">.</span><span class="token function">remove</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>myArrayList<span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token class-name">MyArrayList</span> myArrayList1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">MyArrayList</span><span class="token punctuation">(</span><span class="token number">20</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>myArrayList1<span class="token punctuation">.</span>data<span class="token punctuation">.</span>length<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br><span class="line-number">30</span><br><span class="line-number">31</span><br><span class="line-number">32</span><br><span class="line-number">33</span><br><span class="line-number">34</span><br><span class="line-number">35</span><br><span class="line-number">36</span><br><span class="line-number">37</span><br><span class="line-number">38</span><br><span class="line-number">39</span><br><span class="line-number">40</span><br><span class="line-number">41</span><br><span class="line-number">42</span><br><span class="line-number">43</span><br><span class="line-number">44</span><br><span class="line-number">45</span><br><span class="line-number">46</span><br><span class="line-number">47</span><br><span class="line-number">48</span><br><span class="line-number">49</span><br><span class="line-number">50</span><br><span class="line-number">51</span><br><span class="line-number">52</span><br><span class="line-number">53</span><br><span class="line-number">54</span><br><span class="line-number">55</span><br><span class="line-number">56</span><br><span class="line-number">57</span><br><span class="line-number">58</span><br><span class="line-number">59</span><br><span class="line-number">60</span><br><span class="line-number">61</span><br><span class="line-number">62</span><br><span class="line-number">63</span><br><span class="line-number">64</span><br><span class="line-number">65</span><br><span class="line-number">66</span><br><span class="line-number">67</span><br><span class="line-number">68</span><br><span class="line-number">69</span><br><span class="line-number">70</span><br><span class="line-number">71</span><br><span class="line-number">72</span><br><span class="line-number">73</span><br><span class="line-number">74</span><br><span class="line-number">75</span><br><span class="line-number">76</span><br><span class="line-number">77</span><br><span class="line-number">78</span><br><span class="line-number">79</span><br><span class="line-number">80</span><br><span class="line-number">81</span><br><span class="line-number">82</span><br><span class="line-number">83</span><br><span class="line-number">84</span><br><span class="line-number">85</span><br><span class="line-number">86</span><br><span class="line-number">87</span><br><span class="line-number">88</span><br><span class="line-number">89</span><br><span class="line-number">90</span><br><span class="line-number">91</span><br><span class="line-number">92</span><br><span class="line-number">93</span><br><span class="line-number">94</span><br><span class="line-number">95</span><br><span class="line-number">96</span><br><span class="line-number">97</span><br><span class="line-number">98</span><br><span class="line-number">99</span><br><span class="line-number">100</span><br><span class="line-number">101</span><br><span class="line-number">102</span><br><span class="line-number">103</span><br><span class="line-number">104</span><br><span class="line-number">105</span><br><span class="line-number">106</span><br><span class="line-number">107</span><br><span class="line-number">108</span><br><span class="line-number">109</span><br><span class="line-number">110</span><br><span class="line-number">111</span><br><span class="line-number">112</span><br><span class="line-number">113</span><br><span class="line-number">114</span><br><span class="line-number">115</span><br><span class="line-number">116</span><br><span class="line-number">117</span><br><span class="line-number">118</span><br><span class="line-number">119</span><br></div></div><blockquote><p>ArrayList其他方法如add(int index, E element)，contains(E)可以去看：https://blog.csdn.net/aizhuyanwei/article/details/78493495</p></blockquote> <h2 id="arraylist的线程安全性"><a href="#arraylist的线程安全性" class="header-anchor">#</a> ArrayList的线程安全性</h2> <p>对ArrayList进行添加元素的操作的时候是分两个步骤进行的，即第一步先在object[size]的位置上存放需要添加的元素；第二步将size的值增加1。由于这个过程在多线程的环境下是不能保证具有原子性的，因此ArrayList在多线程的环境下是线程不安全的。</p> <p>具体举例说明：在单线程运行的情况下，如果Size = 0，添加一个元素后，此元素在位置 0，而且Size=1；而如果是在多线程情况下，比如有两个线程，线程 A 先将元素存放在位置0。但是此时 CPU 调度线程A暂停，线程 B 得到运行的机会。线程B也向此ArrayList 添加元素，因为此时 Size 仍然等于 0 （注意哦，我们假设的是添加一个元素是要两个步骤哦，而线程A仅仅完成了步骤1），所以线程B也将元素存放在位置0。然后线程A和线程B都继续运行，都增 加 Size 的值。  那好，现在我们来看看 ArrayList 的情况，元素实际上只有一个，存放在位置 0，而Size却等于 2。这就是“线程不安全”了。</p> <p>如果非要在多线程的环境下使用ArrayList，就需要保证它的线程安全性，通常有两种解决办法：第一，使用synchronized关键字；第二，可以用Collections类中的静态方法synchronizedList();对ArrayList进行调用即可。</p> <h2 id="arraylist和linkedlist、vector的优缺点"><a href="#arraylist和linkedlist、vector的优缺点" class="header-anchor">#</a> ArrayList和LinkedList、Vector的优缺点？</h2> <ul><li><p>ArrayList底层是数组结构，查询快，增删慢，线程不安全，效率高</p></li> <li><p>LinkedList底层是链表数据结构，查询慢，增删快，线程不安全，效率高</p></li> <li><p>Vector底层是数组结构，查询快，增删慢，线程安全，效率低</p></li></ul></div> <footer 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